src: suppress coverity message

Coverity marked a change in 630096b as a constant expression.
However, on platforms where sizeof(int64_t) > sizeof(size_t),
this should not be the case. This commit flags the comparison
as OK to coverity.

PR-URL: https://github.com/nodejs/node/pull/7587
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

src/node_buffer.cc

@@ -187,6 +187,7 @@ inline MUST_USE_RESULT bool ParseArrayIndex(Local<Value> arg,
 
   // Check that the result fits in a size_t.
   const uint64_t kSizeMax = static_cast<uint64_t>(static_cast<size_t>(-1));
+  // coverity[pointless_expression]
   if (static_cast<uint64_t>(tmp_i) > kSizeMax)
     return false;